Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate.
Traditionally, the plate is exposed with an ultraviolet light from a lamp through a separate photomask film having predetermined imaging pattern that is placed between the light source and the plate. Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength, allowing the elimination of the photomask film. Suitable lasers include, for example, infrared lasers (such as laser diode of about 830 nm and NdYAG laser of about 1064 nm), visible lasers (such as frequency-doubled NdYAG laser of about 532 nm and violet laser diode of about 405 nm), and ultraviolet laser (such as ultraviolet laser diode of about 370 nm). Laser sensitive plates generally have higher sensitivity (than conventional plates) because of the limited power of current laser imagers.
Conventionally, the exposed plate is developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas. On-press developable lithographic printing plates have been disclosed in recent years. Such plates can be directly mounted on press after imagewise exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing the reduction of labor and the elimination of hazardous waste. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.
On-press developable plates are designed to be developed on a lithographic printing press, so the non-hardened (or solubilized) areas of the photosensitive layer of the plates are not removed before mounting on press. Therefore, unlike conventionally developed plate which achieves the visible image primarily through the removal of the non-hardened (or solubilized) areas of the photosensitive layer (in addition to possibly also through printout during exposure), the visible image for an on-press developable plate before mounting on press can not be achieved through the removal of the non-hardened (or solubilized) areas of the photosensitive layer. Instead, the visible image for on-press developable plate before mounting on press generally needs to be achieved through the formation of the color contrast between the exposed and non-exposed areas of the photosensitive layer.
Although it is well known that visible image can be achieved by adding certain latent dye (such as leuco crystal violet or other leuco dye) or photo bleachable dye (such as cyanine dye) or utilizing certain resin (such as diazo resin) or initiator (such as diaryliodonium salt which is also a latent dye) in the photosensitive layer so that it changes color in the exposed areas upon imagewise exposure, such visible image is generally much weaker than the image for typical conventionally developed plates. Also, the visible image formed upon the imagewise exposure often fades after exposure to the room light overtime or after storage, due to, for example, the turning on of the unconverted latent dye under room light (such as with leuco crystal violet) or the instability of certain latent dye (such as with a diaryliodonium salt, the printout of which fades with time). In addition, the inventor has observed that addition of the latent dye (such as a leuco dye) in the photosensitive layer generally decreases the photospeed of the plate, especially for a laser sensitive plate. Therefore, it is very difficult to design an on-press developable plate with good visible image.
Before mounting an imagewise exposed plate on press, it is very important for it to have good visible image, so that the pressman can identify and inspect the plate. Accordingly, there has been a desire for an on-press developable plate as well as method which can give good visible image after imagewise exposure (with or without a simple treatment) and before mounting on press.